1. Field of the invention
This invention relates to a microwave oven having a magnetron generating microwaves which are irradiated onto food for heating and cooking it, and more particularly to such a microwave oven wherein a control circuit for the heating operation is provided on a substrate.
2. Description of the prior art
Generally, microwave ovens comprise a magnetron for generating microwaves and a substrate of electronic circuitry including a control circuit for controlling the magnetron. The control circuit is composed of a microcomputer-based electronic circuit in addition to a power transformer, a power circuit and the like.
The power transformer and the power circuit are normally in an on-state. Upon receipt of a key-input for the cooking, the microcomputer controls the magnetron so that it is energized and deenergized in accordance with a program previously stored in it, whereby food contained in a heating chamber is heated.
Some amount of heat is generated by the power transformer and the power circuit since they are normally in the on-state, as described above. Accordingly, the temperature in the vicinity of the substrate of the electronic circuitry is higher than the room temperature. Such an environment provides for a good place for insects breeding in a kitchen, for example, cockroaches, to inhabit.
On the other hand, a number of openings or holes such as cooling louvers are formed in walls of the casing of the microwave oven so that parts enclosed in the casing, such as the magnetron and the high-voltage transformer are cooled. These openings give access to the interior of the casing for the insects such as the cockroaches, which intensifies the tendency for the insects to inhabit the casing interior of the microwave oven.
When the insects such as the cockroaches invade and inhabit the casing interior, particularly, the portion of the casing interior where the substrate of the electronic circuitry is disposed, the substrate is polluted by waste matters excreted from the insects or dead bodies of the insects. These waste matters or dead bodies of the insects cause failure in insulation in electrical parts composing the electronic circuitry and the like. Furthermore, the electrical parts are capacitive-coupled by the waste matters or the dead bodies of the insects. Consequently, the waste matters and the dead bodies of the insects become the cause for malfunction and failure of the microwave oven.
To overcome the above-described deficiency, some countermeasures have been proposed in the prior art. For example, one countermeasure is to close paths through which the insects invade the interior of the microwave oven. Another countermeasure is to cover the portion of the substrate of the electronic circuitry. However, the changes in the mechanical construction resulting from these countermeasures creates additional cost, resulting in increase in the cost of the microwave oven.